Method to control the execution of a shift to a higher gear with a released accelerator pedal in a drivetrain provided with a dual-clutch, servo-assisted transmission

ABSTRACT

A method to control the execution of a shift to a higher gear with a released accelerator pedal in a drivetrain provided with a dual-clutch, servo-assisted transmission, comprising the steps of:
     opening, in a first instant, an outgoing clutch; closing, in the first instant, an incoming clutch; synchronizing, between a second instant and a third instant, a rotation speed of the internal combustion engine with a rotation speed of the incoming clutch, namely with the rotation speed imposed by the gear ratio of the following gear; completely opening, in the third instant, the outgoing clutch; completely closing, in the third instant, the incoming clutch; keeping the torque transmitted by the outgoing clutch constant between the second instant and a fourth instant; and keeping the torque transmitted by the incoming clutch constant between the second instant and the fourth instant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Patent Application claims priority from Italian Patent ApplicationNo. 102019000017543 filed on Sep. 30, 2019, the entire disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a method to control the execution of a shift toa higher gear with a released accelerator pedal in a drivetrain providedwith a dual-clutch, servo-assisted transmission (namely, a gear shift inwhich the following or incoming gear is higher than the previous oroutgoing gear).

PRIOR ART

A drivetrain provided with a dual-clutch, servo-assisted transmissioncomprises a pair of primary shafts, which are coaxial to one another,are independent of one another and are inserted inside one another; twocoaxial clutches, each designed to connect a respective primary shaft toa drive shaft of an internal combustion engine; and at least onesecondary shaft, which transmits the motion to the drive wheels and canbe coupled to the primary shafts by means of respective gear trains,each defining a gear.

During a gear shift, the current gear couples the secondary shaft to aprimary shaft, while the following gear couples the secondary shaft tothe other primary shaft; as a consequence, the gear shift takes place bycrossing the two clutches, namely by opening the clutch associated withthe current gear and by simultaneously closing the clutch associatedwith the following gear.

Currently, a shift to a higher gear with a released accelerator pedalentails opening the outgoing clutch (namely, the clutch associated withthe previous gear), decreasing the rotation speed of the internalcombustion engine due to the braking torque generated by the internalcombustion engine operating in engine braking mode and, finally, closingthe incoming clutch (namely, the clutch associated with the followinggear). In this way, the synchronization (decrease) of the rotation speedof the internal combustion engine with the speed imposed by thefollowing gear (namely, by the incoming clutch) takes place when bothclutches are open, thus causing the internal combustion engine tooperate in engine braking mode (namely, the internal combustion engineis in cut-off condition and operates as engine brake, generating abraking torque).

This mode of execution of a shift to a higher gear with a releasedaccelerator pedal is critical from the control point of view since thestep of adjusting the rotation speed of the internal combustion engineis completely assigned to the sole internal combustion engine, as bothclutches are kept open; it often happens that the rotation speed of theinternal combustion engine decreases too much in order to then besynchronized with a sudden increase, thus generating a first forwardpull and subsequently a strong backward pull in the moment in which therotation speed of the internal combustion engine tries and increaseagain so as to reach the rotation speed of the incoming clutch (as aconsequence, the overall sensation perceived by drivers is not verycomfortable).

U.S. Pat. No. 6,881,171B2 describes a method to control the execution,in a drivetrain provided with a dual-clutch, servo-assistedtransmission, of a gear shift during which the torque at the output ofthe transmission is kept constant or monotonically changing or thelongitudinal acceleration of the vehicle is caused to monotonicallychange.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a method to control theexecution of a shift to a higher gear with a released accelerator pedalin a drivetrain provided with a dual-clutch, servo-assistedtransmission, said method not suffering from the drawbacks discussedabove and, at the same time, being easy and economic to be implemented.

According to the invention there is provided a method to control theexecution of a shift to a higher gear with a released accelerator pedalin a drivetrain provided with a dual-clutch, servo-assistedtransmission, according to the appended claims.

The appended claims describe preferred embodiments of the invention andform an integral part of the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings, showing a non-limiting embodiment thereof, wherein:

FIG. 1 is a schematic plan view of a rear-wheel drive road vehicleprovided with a drivetrain with a dual-clutch, servo-assistedtransmission, which is controlled according to the control method of theinvention;

FIG. 2 is a schematic view of the drivetrain of FIG. 1; and

FIGS. 3 and 4 show the time development of the torques transmitted bythe two clutches of the dual-clutch transmission, of the rotation speedof a drive shaft of the internal combustion engine, of the longitudinaldeceleration of the road vehicle and of the torque generated by theinternal combustion engine during respective shifts to a higher gearcarried out with the control method according to the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, number 1 indicates, as a whole, a road vehicle (inparticular, a car) provided with two front driven (namely, non-drive)wheels 2 and with two rear drive wheels 3. In a front position there isan internal combustion engine 4, which is provided with a drive shaft 5,which produces a torque T_(E), which is transmitted to the drive wheels3 by means of a drivetrain 6. The drivetrain 6 comprises a dual-clutch,servo-assisted transmission 7 arranged in the rear-wheel-drive assemblyand a transmission shaft 8, which connects the drive shaft 5 to an inputof the dual-clutch, servo-assisted transmission 7. The dual-clutch,servo-assisted transmission 7 is connected, in a train-like manner, to aself-locking differential 9, from which a pair of axle shafts 10 start,each integral to a drive wheel 3.

The road vehicle 1 comprises a control unit 11 of the internalcombustion engine 4, which controls the internal combustion engine 4, acontrol unit 12 of the drivetrain 6, which controls the drivetrain 6,and a BUS line 13, which is manufactured, for example, according to theCAN (Car Area Network) protocol, extends to the entire road vehicle 1and allows the two control units 11 and 12 to communicate with oneanother. In other words, the control unit 11 of the internal combustionengine 4 and the control unit 12 of the drivetrain 6 are connected tothe BUS line 13 and, therefore, can communicate with one another bymeans of messages sent through the BUS line 13. Furthermore, the controlunit 11 of the internal combustion engine 4 and the control unit 12 ofthe drivetrain 6 can be directly connected to one another by means of adedicated synchronization cable 14, which is capable of directlytransmitting a signal from the control unit 12 of the drivetrain 6 tothe control unit 11 of the internal combustion engine 4 without thedelays caused by the BUS line 13. Alternatively, the synchronizationcable 14 could be absent and all communications between the two controlunits 11 and 12 could be exchanged using the BUS line 13.

According to FIG. 2, the dual-clutch, servo-assisted transmission 7comprises a pair of primary shafts 15, which are coaxial to one another,independent of one another and inserted inside one another. Furthermore,the dual-clutch, servo-assisted transmission 7 comprises two coaxialclutches 16, each designed to connect a respective primary shaft 15 tothe drive shaft 5 of the internal combustion engine 4 through theinterposition of the transmission shaft 8; each clutch 16 is an oil bathclutch and, hence, is pressure-controlled (i.e. the degree ofopening/closing of the clutch 16 is determined by the pressure of theoil inside the clutch 16); according to an alternative embodiment, eachclutch 16 is a dry clutch and, hence, is position-controlled (i.e. thedegree of opening/closing of the clutch 16 is determined by the positionof a movable element of the clutch 16). The dual-clutch, servo-assistedtransmission 7 comprises one single secondary shaft 17 connected to thedifferential 9 that transmits the motion to the drive wheels 3;according to an alternative and equivalent embodiment, the dual-clutch,servo-assisted transmission 7 comprises two secondary shafts 17, bothconnected to the differential 9.

The dual-clutch, servo-assisted transmission 7 has seven forward gearsindicated with Roman numerals (first gear I, second gear II, third gearIII, fourth gear IV, fifth gear V, sixth gear VI and seventh gear VII)and a reverse gear (indicated with R). The primary shaft 15 and thesecondary shaft 17 are mechanically coupled to one another by aplurality of gear trains, each defining a respective gear and comprisinga primary gear wheel 18 fitted on the primary shaft 15 and a secondarygear wheel 19 fitted on the secondary shaft 17. In order to allow for acorrect operation of the dual-clutch, servo-assisted transmission 7, allodd gears (first gear I, third gear III, fifth gear V, seventh gear VII)are coupled to a same primary shaft 15, whereas all even gears (secondgear II, fourth gear IV and sixth gear VI) are coupled to the otherprimary shaft 15.

Each primary gear wheel 18 is splined to a respective primary shaft 15,so as to always rotate with the primary shaft 15 in an integral manner,and permanently meshes with the respective secondary gear wheel 19; onthe other hand, each secondary gear wheel 19 is mounted on the secondaryshaft 17 in an idle manner. Furthermore, the dual-clutch, servo-assistedtransmission 7 comprises four synchronizers 20, each mounted coaxial tothe secondary shaft 17, arranged between two secondary gear wheels 19and designed to be operated so as to alternatively fit the tworespective secondary gear wheels 19 to the secondary shaft 17 (i.e. soas to alternatively cause the two respective secondary gear wheels 19 tobecome angularly integral to the secondary shaft 17). In other words,each synchronizer 20 can be moved in one direction to fit a secondarygear wheel 19 to the secondary shaft 17 or can be moved in the otherdirection to fit the other secondary gear wheel 19 to the secondaryshaft 17.

The dual-clutch transmission 7 comprises one single secondary shaft 17connected to the differential 9 that transmits the motion to the drivewheels 3; according to an alternative and equivalent embodiment, thedual-clutch transmission 7 comprises two secondary shafts 17, bothconnected to the differential 9.

According to FIG. 1, the road vehicle 1 comprises a passengercompartment housing a driving position for the driver; the drivingposition comprises a seat (which is not shown), a steering wheel 21, anaccelerator pedal 22, a brake pedal 23 and two paddle shifters 24 and25, which control the dual-clutch, servo-assisted transmission 7 and areconnected to the opposite sides of the steering wheel 21. The upshiftpaddle shifter 24 is operated by the driver (by means of a shortpressure) in order to request an upshift (namely, the engagement of anew gear, which is higher than the current gear and contiguous with thecurrent gear), whereas the downshift paddle shifter 25 is operated bythe driver (by means of short pressure) in order to request a downshift(namely, the engagement of a new gear, which is lower than the currentgear and is contiguous with the current gear).

Hereinafter there is a description of the modes of execution of anupshift with a released accelerator pedal 22 from a current, lower gearA to a following, higher gear B (when the accelerator pedal 22 isreleased, the internal combustion engine 4 operates in cut-off conditionand acts as engine brake); namely, the current gear A has a smaller gearratio than the following gear B (hence, given the same speed of the roadvehicle 1, the current gear A causes the internal combustion engine 4 torun more quickly than the following gear B).

In an initial situation (i.e. before the gear shift), an outgoing clutch16B is closed in order to transmit the motion to a primary shaft 15A,which, in turn, transmits the motion to the secondary shaft 17 throughthe current gear A, which is engaged; an incoming clutch 16B, on theother hand, is open and, hence isolates a primary shaft 15B from thetransmission shaft 8. Before beginning the upshift, the following gear Bis engaged in order to connect, through the gear B, the primary shaft15B to the secondary shaft 17. When the driver sends the gear shiftcommand, the gear shift is carried out by opening the outgoing clutch16A in order to disconnect the primary shaft 15A (hence, the gear A)from the transmission shaft 8 (i.e. from the drive shaft 5 of theinternal combustion engine 4) and, (more or less) simultaneously, byclosing the incoming clutch 16B in order to connect the primary shaft15B (hence, the gear B) to the transmission shaft 8 (i.e. to the driveshaft 5 of the internal combustion engine 4).

FIG. 3 shows the ways in which a shift to a higher gear is carried out,the driver sending the upshift command by acting upon the upshift paddleshifter 24 while the accelerator pedal 22 is released. FIG. 3 shows,starting from the top:

a first diagram showing the time development of the rotation speed ω_(E)of the internal combustion engine 4, the rotation speed ω_(A) of theoutgoing clutch 16A and the rotation speed ω_(B) of the incoming clutch16B; a second diagram showing the time development of the torques T_(A)and T_(B) transmitted by the two clutches 16A and 16B;

a third diagram showing the time development of the torque T_(E)generated by the internal combustion engine 4 (before and after theupshift the internal combustion engine 4 is in cut-off condition and,hence, operates in engine braking mode generating a negative torqueT_(E)); and

a fourth diagram showing the time development of the longitudinalacceleration α of the vehicle 1 (it should be pointed out that thelongitudinal acceleration α of the vehicle 1 always is negative, namelythe vehicle 1 is slowing down as the internal combustion engine 4 isgenerating a negative torque T_(E), namely is braking, thus operating inengine braking mode).

As soon as the control unit 12 of the drivetrain 6 receives the gearshift command from the driver (instant t₀), the control unit 12 of thedrivetrain immediately starts filling the incoming clutch 16B, namely itimmediately starts feeding oil under pressure into the incoming clutch16B; indeed, the incoming clutch 16B associated with the following gearB can transmit a significant torque to the rear drive wheels 3 only whenthe filling with oil under pressure has been completed and, hence, theoil under pressure, for it cannot occupy further volume inside theincoming clutch 16B, exerts a thrust that packs the discs of theincoming clutch 16B. As a consequence, before the incoming clutch 16Bassociated with the following gear B can actually start transmitting asignificant torque to the rear drive wheels 3, it is necessary to waitfor a given delay time interval (typically ranging from 80 to 220thousandths of second), during which the filling of the incoming clutch16B with oil is completed. The completion of the filling of the incomingclutch 16B is normally monitored through a pressure sensor, whichdetects the pressure of the oil inside the incoming clutch 16B: when thepressure of the oil inside the incoming clutch 16B exceeds apredetermined threshold, this means that the inner volume of theincoming clutch 16B was completely filled and, hence, the oil inside theclutch 16B starts compressing. As a consequence, the instant t₁ in which(after the delay time has elapsed) the incoming clutch 16B is filledwith oil and is ready to transit a significant torque is establishedwhen the pressure of the oil inside the incoming clutch 16B exceeds thepredetermined threshold.

From the instant t₀, in which the control unit 12 of the drivetrainimmediately starts closing the incoming clutch 16B, to the instant t₁,in which, after the delay time has elapsed, the incoming clutch 16B isfilled with oil and is ready to transmit a significant torque, nothinghappens to the dynamic of the road vehicle 1, i.e. the entire torqueT_(E) generated by the internal combustion engine 4 (which is a negativetorque T_(E), namely a braking torque T_(E), since the internalcombustion engine 4 is in cut-off condition and, hence, operates asengine brake) is entirely transmitted by the outgoing clutch 16A, likebefore the beginning of the gear shift. In the instant t₁, the incomingclutch 16B starts transmitting a torque T_(B) (namely, the torque T_(B)starts increasing) and, at the same time, the outgoing clutch 16A isordered to open (namely, the torque T_(A) starts decreasing); it shouldbe pointed out that the opening of the outgoing clutch 16A associatedwith the current gear A takes place with no delay as the outgoing clutch16A is already filled with oil under pressure and, in this phase, itsimply needs to be emptied from part of the oil by opening a solenoidvalve (whose action, thus, is instantaneous).

Between the instants t₁ and t₂ there is a partial transfer of torquebetween the clutches 16A and 16B: the torque T_(A) transmitted by theoutgoing clutch 16A decreases with a linear ramp between the instants t₁and t₂ and in the instant t₁ the torque transmitted by the incomingclutch 16B increases in a step-like manner (by a quantity which issmaller than the overall decrease in the torque T_(A) transmitted by theoutgoing clutch 16A). Subsequently, between the instants t₂ and t₃, thetorques T_(A) and T_(B) transmitted by the two clutches 16A and 16Bremain constant and, in this amount of time, the torque T_(A)transmitted by the outgoing clutch 16A is greater than the torque T_(B)transmitted by the incoming clutch 16B.

Subsequently, between the instants t₃ and t₄ there is a completetransfer of torque between the two clutches 16A and 16B, i.e. the torquetransmitted by the outgoing clutch 16A progressively decreases up tozero (the outgoing clutch 16A is opened by means of a linear ramp) and,at the same time, the torque transmitted by the incoming clutch 16Bprogressively increases (the incoming clutch 16B is closed by means of alinear ramp), thus determining a crossing between the two clutches 16Aand 16B. The clutches 16A and 16B are opened and closed by means oflinear ramps, namely the respective torques T_(A) and T_(B) change overtime (decreasing and increasing) with linear variation laws.

The outgoing clutch 16A is completely opened in the same amount of timeneeded to completely close the incoming clutch 16B; therefore, in theinstant t₄ the outgoing clutch 16A is completely open (i.e. does nottransmit a torque any longer), whereas the incoming clutch 16B iscompletely closed (i.e. transmits the entire torque T_(E) of theinternal combustion engine 4). Between the instants t₁ and t₄ there isthe shifting time, during which the torque transmitted by the outgoingclutch 16A decreases until it becomes zero and, simultaneously, thetorque transmitted by the incoming clutch 16B increases until it reachesthe torque T_(E) generated by the internal combustion engine 4 (asalready mentioned above, the internal combustion engine 4 is in cut-offcondition and, hence, operates as engine brake, thus generating anegative torque T_(E)), namely during which the outgoing clutch 16Aseparates itself from the drive wheels 3 and the incoming clutch 16Bgets connected to the drive wheels 3.

The rotation speed ω_(E) of the internal combustion engine 4 is equal tothe rotation speed ω_(A) imposed by the gear ratio of the current gear Abefore the gear shift until the instant t₂, it progressively decreasestowards the rotation speed ω_(B) imposed by the gear ratio of thefollowing gear during the gear shift and is equal to the rotation speedω_(B) after the gear shift.

Between the instants t₂ and t₄ there is the synchronization time, duringwhich the rotation speed ω_(E) of the internal combustion engine 4decreases from the rotation speed ω_(A) imposed by the gear ratio of thecurrent gear A to the rotation speed ω_(B) imposed by the gear ratio ofthe following gear B, namely the rotation speed ω_(E) is synchronizedwith the rotation speed ω_(B). In order to decrease the rotation speedω_(E) of the internal combustion engine 4, the negative (namely,braking) torque T_(E) generated by the internal combustion engine 4 isused between the instants t₂ and t₄.

The longitudinal acceleration α of the vehicle 1 is approximatelyconstant and equal to the value α_(A) (which is negative, since thevehicle is slowing down) immediately before the gear shift and isapproximately constant and equal to the value α_(B) (which is negative,since the vehicle is slowing down, and smaller than the value α_(B) inabsolute value) immediately after the gear shift. During the gear shift,the longitudinal acceleration of the vehicle 1 progressively increasesfrom the initial value α_(A) to the final value α_(B).

In the embodiment shown in FIG. 3, the internal combustion engine 4 isin cut-off condition before the upshift (hence, it operates as enginebrake generating a braking torque T_(E)) and continues being in acut-off condition even after the upshift; namely, before, during andafter the upshift the driver keeps the accelerator pedal 22 completelyreleased. However, it can happen that the driver, after having requestedthe upshift (namely, after the instant t₀), decides to press theaccelerator pedal 22, thus requesting that the internal combustionengine 4 generates a positive torque T_(E) (namely, a drive torque);this situation is present in the embodiment shown in FIG. 4, in which inthe instant t₅ (for example placed between the instant t₂ and theinstant t₃) the driver, after having requested the upshift (namely,after the instant to), presses the accelerator pedal 22. Obviously, thecontrol unit 12 of the drivetrain 6 must anyway complete the upshiftthat has already started, but can try and give an immediate response tothe new (and unpredicted) request of the driver, so as to give thedriver a feeling of extreme responsiveness; as a consequence, startingfrom the instant t₃ (subsequent to the instant t₅), the control unit 12of the drivetrain 6 requests that the control unit 11 of the internalcombustion engine 4 increases the torque T_(E) generated by the internalcombustion engine 4 (according to the request of the driver, who pressedthe accelerator pedal 22) and, at the same time, accelerates the closingof the incoming clutch 16B (namely, makes the closing of the incomingclutch 16B faster), which, obviously, now has to transmit, as a whole, agreater torque T_(B) due to the fact that the internal combustion engine4 was turned on.

In other words, the control unit 12 of the drivetrain 6 increases, incase the accelerator pedal 22 is pressed by the driver during theupshift in the instant t₅ (which is subsequent to the instant t₁ andprior to the instant t₄), the torque T_(E) generated by the internalcombustion engine 4 starting from the instant t₃, if the instant t₅ isprior to the instant t₃ (according to FIG. 4), or starting from theinstant t₅, if the instant t₅ is subsequent to the instant t₃;furthermore, the control unit 12 of the drivetrain 6 accelerates, incase the accelerator pedal 22 is pressed by the driver during theupshift in the instant t₅ (which is subsequent to the instant t₁ andprior to the instant t₄), the closing of the incoming clutch 16Bstarting from the instant t₃, if the instant t₅ is prior to the instantt₃ (according to FIG. 4), or starting from the instant t₅, if theinstant t₅ is subsequent to the instant t₃.

The control method described above has different advantages.

First of all, the method to control the execution of a shift to a highergear described above is very comfortable, as, since the transmission ofthe torque to the drive wheels is never interrupted, it determines acomfortable longitudinal acceleration profile of the road vehicle:indeed, the longitudinal acceleration of the road vehicle graduallyshifts, without gradient inversion, from an initial deceleration to afinal deceleration, which is smaller than the previous one (in theupshift the gear ratio gets longer and, therefore, the engine brake hasa less incisive effect on the dynamic of the road vehicle 1); in thisway, the driver always has a feeling of continuous deceleration.

Furthermore, the method to control the execution of a shift to a highergear described above does not generate any perceivable metal noise, asthe two clutches 16A and 16B never are both open at the same time and,hence, the drivetrain 6 always is “under stress”; namely, backlashes aresignificantly reduced and, as a consequence, so are noises, since thegear trains of the drivetrain 6 always are “under stress”.

The method to control the execution of a shift to a higher geardescribed above also allows for the implementation of a new strategy(also called “change of mind”), which allows the torque T_(B) of theincoming clutch 16B to be adjusted if, during the upshift, the driverpresses the accelerator pedal 22, thus allowing for a more prompt andquick response of the road vehicle 1 to the commands of the driver.

Finally, the control method described above is easy and economic to beimplemented, since it does not require the installation of additionalphysical components and does not call for an expansion of the controlunit 12 of the drivetrain 6, since no additional calculation ability isneeded.

LIST OF THE REFERENCE NUMBERS OF THE FIGURES

1 road vehicle

2 front wheels

3 rear wheels

4 engine

5 drive shaft

6 drivetrain

7 transmission

8 transmission shaft

9 differential

10 axle shafts

11 engine control unit

12 drivetrain control unit

13 BUS line

14 synchronization cable

15 primary shafts

16 clutches

17 secondary shaft

18 primary gear wheel

19 secondary gear wheel

20 synchronizers

21 steering wheel

22 accelerator pedal

23 brake pedal

24 upshift paddle shifter

25 downshift paddle shifter

ω_(E) rotation speed

ω_(A) rotation speed

ω_(B) rotation speed

T_(E) torque

T_(A) torque

T_(B) torque

α acceleration

t₀ time instant

t₁ time instant

t₂ time instant

t₃ time instant

t₄ time instant

t₅ time instant

1) A method to control the execution of a shift to a higher gear with areleased accelerator pedal (22) in a drivetrain (6) provided with adual-clutch, servo-assisted transmission (7), so as to shift from acurrent gear (A) to a following gear (B), which is shorter than thecurrent gear (A); the drivetrain (6) comprises a dual-clutch,servo-assisted transmission (7) having two primary shafts (15); at leastone secondary shaft (17) connected to drive wheels (3); and two clutches(16A, 16B), each interposed between a drive shaft (5) of an internalcombustion engine (4) and a corresponding primary shaft (15); thecontrol method comprises the steps of: opening, in a first instant (t₁),an outgoing clutch (16A) associated with the current gear (A); closing,in the first instant (t₁), an incoming clutch (16B) associated with thefollowing gear (B); synchronizing, between a second instant (t₂), whichis subsequent to the first instant (t₁), and a third instant (t₄), whichis subsequent to the second instant (t₂), a rotation speed (ω_(E)) ofthe internal combustion engine (4) with a rotation speed (ω_(B)) of theincoming clutch (16B), namely with the rotation speed (ω_(B)) imposed bythe gear ratio of the following gear (B); completely opening, in thethird instant (t₄), the outgoing clutch (16A); completely closing, inthe third instant (t₄), the incoming clutch (16A); keeping the torque(T_(A)) transmitted by the outgoing clutch (16A) constant between thesecond instant (t₂) and a fourth instant (t₃), which is subsequent tothe second instant (t₂) and prior to the third instant (t₄); and keepingthe torque (T_(B)) transmitted by the incoming clutch (16B) constantbetween the second instant (t₂) and the fourth instant (t₃). 2) Thecontrol method according to claim 1, wherein, between the second instant(t₂) and the fourth instant (t₃), the torque (TB) transmitted by theincoming clutch (16B) is smaller than the torque (TA) transmitted by theoutgoing clutch (16A). 3) The control method according to claim 1,wherein, between the fourth instant (t₃) and the third instant (t₄), thetorque (T_(B)) transmitted by the incoming clutch (16B) is increasedwith a first linear ramp. 4) The control method according to claim 3,wherein, between the fourth instant (t₃) and the third instant (t₄), thetorque (T_(A)) transmitted by the outgoing clutch (16A) is decreasedwith a second linear ramp. 5) The control method according to claim 1,wherein, in the first instant (t₁), the torque (T_(B)) transmitted bythe incoming clutch (16B) is increased in a stepped manner. 6) Thecontrol method according to claim 1, wherein, between the first instant(t₁) and the second instant (t₂), the torque (T_(B)) transmitted by theincoming clutch (16B) is decreased with a third linear ramp. 7) Thecontrol method according to claim 1, wherein the internal combustionengine (4) is in cut-off condition before the shift to a higher gear andis in cut-off condition also after the shift to a higher gear. 8) Thecontrol method according to claim 7, wherein, before, during and afterthe shift to a higher gear, a driver keeps the accelerator pedal (22)completely released. 9) The control method according to claim 1 andcomprising the further steps of: detecting a pressing of the acceleratorpedal (22) exerted by a driver during the shift to a higher gear;increasing, in case an accelerator pedal (22) is pressed by a driverduring the shift to a higher gear in a fifth instant (t₅) subsequent tothe first instant (t₁) and prior to the third instant (t₄), the torque(T_(E)) generated by the internal combustion engine (4) starting fromthe fourth instant (t₃), if the fifth instant (t₅) is prior to thefourth instant (t₃), or starting from the fifth instant (t₅), if thefifth instant (t₅) is subsequent to the fourth instant (t₃). 10) Thecontrol method according to claim 1 and comprising the further steps of:detecting a pressing of the accelerator pedal (22) exerted by a driverduring the shift to a higher gear; accelerating, in case an acceleratorpedal (22) is pressed by a driver during the shift to a higher gear in afifth instant (t₅) subsequent to the first instant (t₁) and prior to thethird instant (t₄), the closing of the incoming clutch (16B) startingfrom the fourth instant (t₃), if the fifth instant (t₅) is prior to thefourth instant (t₃), or starting from the fifth instant (t₅), if thefifth instant (t₅) is subsequent to the fourth instant (t₃).